Friction clutches are already known in which the friction members, for example friction plates, are urged into engagement with one another by a piston operative in a cylinder. Upon initiating an operation of such clutches, the pressure of the pressure medium often builds up suddenly, so that these clutches often move with a considerable and sudden force (German Pat. No. 862,237). It is known to diminish the impact or grab thereby occuring by providing in the hydraulic unit a pressure fluid chamber or storage cylinder. This increases the expense of the hydraulic unit and causes delay in operation (German Pat. No. 933,186).
Furthermore a friction clutch is known in which for damping the initially force auxiliary pistons are provided which form inlet pressure chambers and each such auxiliary piston becomes effective before the main engaging element, through which a graduated coupling power is effected. This, however, results in a very complicated construction (German AS No. 1,268,451).
Finally a hydraulically operated clutch unit is known in which engagement takes place quickly through resilient load absorption. For this purpose a partition wall defining a counter-pressure chamber is provided within the movable coupling housing, which partition wall divides into two chambers the space defined by the housing. These two chambers are, when the clutch is uncoupled, always maintained at a relatively low pressure. Openings and valves are provided in the counter-pressure chamber, which valves are urged by the low pressure liquid into such positions as to permit the stream of liquid to flow through the openings. This clutch also is very expensive (German OS No. 1,400,421 ).